Electronic controls enable motor vehicle engines to operate in ways that accomplish multiple goals, such as improving vehicle driveability, increasing fuel mileage, and lowering tailpipe emissions. For any of various reasons, an engine will experience transient operation while in use. Some transients are less severe, others more severe.
Transients, especially severe ones, can upset desired engine control strategy, leading to undesired effects such as engine stumbling and the generation of increased amounts of certain tailpipe emissions. Consequently, engine control strategies typically take transients into account in some way.
One way in which an engine transient can arise is by operation of the accelerator pedal. If the accelerator is rapidly depressed, the driver is typically calling for increased engine torque to accelerate the engine and/or prevent the engine from decelerating in response to an increase in engine load. When it is desired to slow a motor vehicle being propelled by an internal combustion engine, the driver typically releases the accelerator pedal. That action alone will cause the vehicle to slow due to various forces acting on the vehicle. Driver action may also include applying the vehicle service brakes, depending on the amount of braking needed.
During such transients, certain variable parameters related to engine operation experience transients themselves, including EGR rate, and in the case of a turbocharged engine, turbocharger boost.